Electrochromic devices typically utilize a combination of two types of electrochromic materials, one of which becomes optically less transmissive (e.g., takes on color) in its electrochemically oxidized state while the other becomes optically less transmissive (e.g., takes on color) in its electrochemically reduced state. For example, Prussian blue assumes a blue color in its electrochemically oxidized state and becomes colorless by reduction while tungsten trioxide WO3, assumes a blue color in its electrochemically reduced state and becomes colorless by oxidation. When the two are used as separate electrochromic layers separated by an ion conductor layer in a multi-layer stack, the stack may be reversibly cycled between a blue color (when the Prussian blue material is in its electrochemically oxidized state and tungsten trioxide is in its reduced state) and a transparent state (when the Prussian blue material is in its electrochemically reduced state and tungsten trioxide is in its electrochemically oxidized state) by application of an appropriate voltage across the stack.
Faradaic losses in reversible electrochromic devices can degrade the performance of reversible electrochromic devices. For example, a faradaic loss may be caused during cycling by a reaction between the electrolyte and an oxidizing electrode surface, by a photochemical oxidation reaction, or by any of a range of other spurious oxidation mechanisms involving water, oxygen, and/or a component of an ion conducting material (e.g., an ion conductor layer). These faradaic losses can, in turn, result in a corresponding change in the oxidation state of an electrochromic material in the electrochromic device. The faradaic losses can occur in the electrochromic material that becomes optically less transmissive in its electrochemically oxidized state, the electrochromic material that becomes optically less transmissive in its electrochemically reduced state, or both. Over time and repeated cycling, the accumulated faradaic losses can cause a drift in the range of optical transmissivities achievable for the device within the desired operating voltage range for the device.
In certain types of electrochromic devices, durability is a major challenge. As the device ages the performance suffers. The transmission in the colored and bleached states can change, the capacity (charge stored in the device in a given state) can change, and the ratio of the transmittance of the device in the bleached state versus the colored state over the visible range of the electromagnetic spectrum can also change. These changes can be large, and easily perceptible to the user of the electrochromic device. The rates of degradation can also be affected by many factors, including but not limited to temperature, applied bias ranges, rate of switching, and intensity and duration of incident solar radiation.
What is therefore desired are electrochromic device architectures, materials, and control schemes that enable an electrochromic device with faradaic losses (i.e., spurious oxidation and/or reduction) to maintain the electrochromic properties throughout the lifetime of the device. Furthermore, the device architectures, materials and control schemes employed should be able to be readily manufacturable into commercially viable products.